1. Field of the Invention
The present invention relates in general to evaporative emission control systems for automotive vehicles and more particularly to an apparatus for determining if a leak is present in an evaporative emission control system for an automotive vehicle. The present invention further relates to a method of detecting such a leak.
2. Description of the Related Art
A prior art evaporative emission control system for an automotive vehicle prevents emission of evaporative fuel to the open air by introducing the fuel vapors produced in a fuel tank to a canister so that the fuel vapors are temporarily absorbed by the canister, and supplying the collected fuel vapors to an intake system of an engine together with fresh air drawn into the canister through its atmospheric vent, as disclosed in Japanese Patent Provisional Publication No. 5-215020.
In the meantime, if there should occur, in the above described evaporative emission control system, a crack and the like in a fuel vapor flow passage extending from the fuel tank through the canister to a purge valve, the fuel vapors leak, resulting in that the evaporative emission control system is no more operative to produce an expected effect of preventing evaporative emission sufficiently.
Thus, it has been proposed such a leak detection method for detecting if a leak is present in the fuel vapor flow passage or not.
By the method, a current through a motor driven air pump (hereinafter also referred to as pump operating current) when air is forced to pass a reference orifice of a reference bore size by the air pump is detected and determined as a criterion or judgment level. On the other hand, a pump operating current at the time air is forcedly transmitted to the fuel vapor flow passage of the evaporative emission control system by the air pump while bypassing the reference orifice is measured and determined as a leak level. By comparing the leak level with the judgement level, it is determined that a leak is present in the evaporative emission control system when the leak level is smaller than the judgement level.
This method enables to detect a leak accurately and assuredly even when the leak results from a small hole in the fuel vapor flow passage, i.e., even when the leak is small.
However, the above described method has a problem that the pressure of evaporative fuel in the fuel tank is high when the temperature of fuel in the fuel tank is high, so the pump operating current is increased by the influence of increase of pressure in the fuel tank even when the fuel vapor flow passage has such a hole or the like that will cause a detectable leak, resulting in the possibility that a leak that is actually present is erroneously judged as no leak. Further, in case the residual quantity of fuel in the fuel tank (hereinafter also referred to as tank residual) is large, such a possibility is enhanced.